Deoxycytidine is a more effective inhibitor of cell growth and colony formation of the human leukemia HL-60 and K-562 than is thymidine and inhibits tritiated nucleoside incorporation. This action is through a direct effect on intracellular nucleotide pool size. In combination with cytosine arabinoside, deoxyguanosine effects the cell kill and incorporation of cytosine arabinoside into cellular DNA. Patients with acute Myelocytic leukemia show evidence of herpes virus like thymidine kinase in both their bone marrow and peripheral blood.